1. Field of the Invention
The present invention relates to steel-sheet continuous annealing equipment and a method for operating such steel-sheet continuous annealing equipment.
2. Description of the Related Art
To enhance the processability of cold-rolled steel sheets, cold-rolled steel sheets are heat-treated with a continuous annealing furnace having, in sequence, a heating zone, a soaking zone, first and second cooling zones, a reheating zone, an overaging zone, and a final cooling zone.
When mild steel sheets or common high-tensile steel sheets are produced by performing a heat treatment with continuous annealing equipment, the time for which steel sheets are held at a temperature after being reheated is 100 seconds or more. Stated another way, after being reheated, steel sheets are held at the temperature for 100 seconds or more.
High-tensile steel products have been demanded to have better characteristics (higher strength, higher elongation, and higher λ) year after year. With this trend, methods for producing high-tensile steel products have become complicated.
For example, as disclosed in Japanese Unexamined Patent Application Publication No. 2008-297592, steel sheets that are used for producing lower weight automobiles or the like and have high strength, high elongation, and a high hole-expansion ratio such as high-tensile steel sheets having a strength of 1,000 MPa, an elongation of 10%, and a hole-expansion ratio of 100% are produced with an additional step such as pre-annealing of steel sheets or addition of a constraint on the coiling temperature in hot rolling. However, such an additional step of pre-annealing steel sheets increases the cost of the production process. To decrease the coiling temperature, strong cooling is necessary and there are many technical requirements to be met to realize practical use, for example, prevention of deformation of steel sheets and achievement of a uniform temperature distribution during coiling of the steel sheets.